Here are the rationale i read on.
Reduces the user experience complexity of adding ERC20 tokens
Allows ERC20 transfer transactions to be easily detected and have the correct value shown on wallets (incl hardware wallets) without requiring pre-setup
Ensures that in any on-chain exchange, the price as expressed in computer units (ie. wei or equivalent) is the same number as the price as expressed in human units (ie. ether or equivalent), reducing the risk of confusion or bugs